​CRYPTOGRAPHY (from Gr. κρύπτος, hidden, and γράφειν,
to write), or writing in cipher, called also steganography (from
Gr. στεγάνη, a covering), the art of writing in such a way as to
be incomprehensible except to those who possess the key to the
system employed. The unravelling of the writing is called
deciphering. Cryptography having become a distinct art,
Bacon (Lord Verulam) classed it (under the name ciphers) as
a part of grammar. Secret modes of communication have been in
use from the earliest times. The Lacedemonians had a method
called the scytale, from the staff (σκυτάλη) employed in constructing
and deciphering the message. When the Spartan ephors
wished to forward their orders to their commanders abroad, they
wound slantwise a narrow strip of parchment upon the σκυτάλη
so that the edges met close together, and the message was then
added in such a way that the centre of the line of writing was
on the edges of the parchment. When unwound the scroll
consisted of broken letters; and in that condition it was
despatched to its destination, the general to whose hands it
came deciphering it by means of a σκυτάλη exactly corresponding
to that used by the ephors. Polybius has enumerated other
methods of cryptography.

The art was in use also amongst the Romans. Upon the
revival of letters methods of secret correspondence were introduced
into private business, diplomacy, plots, &c.; and as the
study of this art has always presented attractions to the
ingenious, a curious body of literature has been the result.

John Trithemius (d. 1516), the abbot of Spanheim, was the
first important writer on cryptography. His Polygraphia,
published in 1518, has passed through many editions, and has
supplied the basis upon which subsequent writers have worked.
It was begun at the desire of the duke of Bavaria; but
Trithemius did not at first intend to publish it, on the ground
that it would be injurious to public interests. A Steganographia
published at Lyons (? 1551) and later at Frankfort (1606), is
also attributed to him. The next treatises of importance were
those of Giovanni Battista della Porta, the Neapolitan mathematician,
who wrote De furtivis litterarum notis, 1563; and of
Blaise de Vigenere, whose Traité des chiffres appeared in Paris,
1587. Bacon proposed an ingenious system of cryptography
on the plan of what is called the double cipher; but while thus
lending to the art the influence of his great name, he gave an
intimation as to the general opinion formed of it and as to the
classes of men who used it. For when prosecuting the earl of
Somerset in the matter of the poisoning of Overbury, he urged
it as an aggravation of the crime that the earl and Overbury
“had cyphers and jargons for the king and queen and all the
great men,—things seldom used but either by princes and their
ambassadors and ministers, or by such as work or practise against
or, at least, upon princes.”

Other eminent Englishmen were afterwards connected with
the art. John Wilkins, subsequently bishop of Chester, published
in 1641 an anonymous treatise entitled Mercury, or The Secretand Swift Messenger,—a small but comprehensive work on the
subject, and a timely gift to the diplomatists and leaders of
the Civil War. The deciphering of many of the royalist papers
of that period, such as the letters that fell into the hands of the
parliament at the battle of Naseby, has by Henry Stubbe been
charged on the celebrated mathematician Dr John Wallis
(Athen. Oxon. iii. 1072), whose connexion with the subject of
cipher-writing is referred to by himself in the Oxford edition of
his mathematical works, 1689, p. 659; as also by John Davys.
Dr Wallis elsewhere states that this art, formerly scarcely known
to any but the secretaries of princes, &c., had grown very common
and familiar during the civil commotions, “so that now there is
scarce a person of quality but is more or less acquainted with it,
and doth, as there is occasion, make use of it.” Subsequent
writers on the subject are John Falconer (Cryptomenysis patefacta),
1685; John Davys (An Essay on the Art of Decyphering:in which is inserted a Discourse of Dr Wallis), 1737; Philip
Thicknesse (A Treatise on the Art of Decyphering and of Writingin Cypher), 1772; William Blair (the writer of the comprehensive
article “Cipher” in Rees’s Cyclopaedia), 1819; and G. von
Marten (Cours diplomatique), 1801 (a fourth edition of which
appeared in 1851). Perhaps the best modern work on this
subject is the Kryptographik of J. L. Klüber (Tübingen, 1809),
who was drawn into the investigation by inclination and official
circumstances. In this work the different methods of cryptography
are classified. Amongst others of lesser merit who
have treated of this art may be named Gustavus Selenus (i.e.
Augustus, duke of Brunswick), 1624; Cospi, translated by
Niceron in 1641; the marquis of Worchester, 1659; Kircher,
1663; Schott, 1665; Ludwig Heinrich Hiller, 1682; Comiers;
1690; Baring, 1737; Conrad, 1739, &c. See also a paper on
Elizabethan Cipher-books by A. J. Butler in the Bibliographical
Society’s Transactions, London, 1901.

Schemes of cryptography are endless in their variety. Bacon
lays down the following as the “virtues” to be looked for in
them:—“that they be not laborious to write and read; that
they be impossible to decipher; and, in some cases, that they
be without suspicion.” These principles are more or less disregarded
by all the modes that have been advanced, including
that of Bacon himself, which has been unduly extolled by his
admirers as “one of the most ingenious methods of writing in
cypher, and the most difficult to be decyphered, of any yet
contrived” (Thicknesse, p. 13).

The simplest and commonest of all the ciphers is that in which
the writer selects in place of the proper letters certain other
letters in regular advance. This method of transposition was
used by Julius Caesar. He, “per quartam elementorum literam,”
wrote d for a, e for b, and so on. There are instances of this
arrangement in the Jewish rabbis, and even in the sacred writers.
An illustration of it occurs in Jeremiah (xxv. 26), where the
prophet, to conceal the meaning of his prediction from all but
the initiated, writes Sheshak instead of Babel (Babylon), the
place meant; i.e. in place of using the second and twelfth letters
of the Hebrew alphabet (b, b, l) from the beginning, he wrote
the second and twelfth (sh, sh, k) from the end. To this kind of
cipher-writing Buxtorf gives the name Athbash (from a the first
letter of the Hebrew alphabet, and th the last; b the second from
the beginning, and h the second from the end). Another Jewish
cabalism of like nature was called Albam; of which an example
is in Isaiah vii. 6, where Tabeal is written for Remaliah. In its
adaptation to English this method of transposition, of which
there are many modifications, is comparatively easy to decipher.
A rough key may be derived from an examination of the respective
quantities of letters in a type-founder’s bill, or a printer’s
“case.” The decipherer’s first business is to classify the letters
of the secret message in the order of their frequency. The letter
that occurs oftenest is e; and the next in order of frequency is t.
The following groups come after these, separated from each other
by degrees of decreasing recurrence:—a, o, n, i; r, s, h; d, l;
c, w, u, m; f, y, g, p, b; v, k; x, q, j, z. All the single letters must
be a, I or O. Letters occurring together are ee, oo, ff, ll, ss, &c.
The commonest words of two letters are (roughly arranged in
the order of their frequency) of, to, in, it, is, be, he, by, or, as,
at, an, so, &c. The commonest words of three letters are the
and and (in great excess), for, are, but, all, not, &c.; and of four
letters—that, with, from, have, this, they, &c. Familiarity with
the composition of the language will suggest numerous other
points that are of value to the decipherer. He may obtain other
hints from Poe’s tale called The Gold Bug. As to messages in the
continental languages constructed upon this system of transposition,
rules for deciphering may be derived from Breithaupt’s Ars decifratoria (1737), and other treatises.

Bacon remarks that though ciphers were commonly in letters and alphabets yet they might be in words. Upon this basis codes have been constructed, classified words taken from dictionaries being made to represent complete ideas. In recent
​years such codes have been adapted by merchants and others to
communications by telegraph, and have served the purpose not
only of keeping business affairs private, but also of reducing
the excessive cost of telegraphic messages to distant markets.
Obviously this class of ciphers presents greater difficulties to
the skill of the decipherer.

Figures and other characters have been also used as letters;
and with them ranges of numerals have been combined as the
representatives of syllables, parts of words, words themselves,
and complete phrases. Under this head must be placed the
despatches of Giovanni Michael, the Venetian ambassador to
England in the reign of Queen Mary, documents which have only
of late years been deciphered. Many of the private letters
and papers from the pen of Charles I. and his queen, who were
adepts in the use of ciphers, are of the same description. One of
that monarch’s letters, a document of considerable interest, consisting
entirely of numerals purposely complicated, was in 1858
deciphered by Professor Wheatstone, the inventor of the ingenious
crypto-machine, and printed by the Philobiblon Society.
Other letters of the like character have been published in the
First Report of the Royal Commission on Historical Manuscripts
(1870). In the second and subsequent reports of the same commission
several keys to ciphers have been catalogued, which
seem to refer themselves to the methods of cryptography under
notice. In this connexion also should be mentioned the “characters,”
which the diarist Pepys drew up when clerk to Sir
George Downing and secretary to the earl of Sandwich and to
the admiralty, and which are frequently mentioned in his journal.
Pepys describes one of them as “a great large character,” over
which he spent much time, but which was at length finished,
25th April 1660; “it being,” says he, “very handsomely done
and a very good one in itself, but that not truly alphabetical.”

Shorthand marks and other arbitrary characters have also
been largely imported into cryptographic systems to represent
both letters and words, but more commonly the latter. This
plan is said to have been first put into use by the old Roman poet
Ennius. It formed the basis of the method of Cicero’s freedman,
Tiro, who seems to have systematized the labours of his predecessors.
A large quantity of these characters have been
engraved in Gruter’s Inscriptiones. The correspondence of
Charlemagne was in part made up of marks of this nature. In
Rees’s Cyclopaedia specimens were engraved of the cipher used
by Cardinal Wolsey at the court of Vienna in 1524, of that used
by Sir Thomas Smith at Paris in 1563, and of that of Sir Edward
Stafford in 1586; in all of which arbitrary marks are introduced.
The first English system of shorthand—Bright’s Characterie,
1588—almost belongs to the same category of ciphers. A
favourite system of Charles I., used by him during the year 1646,
was one made up of an alphabet of twenty-four letters, which
were represented by four simple strokes varied in length, slope
and position. This alphabet is engraved in Clive’s Linear Systemof Shorthand (1830), having been found amongst the royal manuscripts
in the British Museum. An interest attaches to this
cipher from the fact that it was employed in the well-known
letter addressed by the king to the earl of Glamorgan, in which
the former made concessions to the Roman Catholics of Ireland.

Complications have been introduced into ciphers by the employment
of “dummy” letters,—“nulls and insignificants,”
as Bacon terms them. Other devices have been introduced to
perplex the decipherer, such as spelling words backwards, making
false divisions between words, &c. The greatest security against
the decipherer has been found in the use of elaborate tables of
letters, arranged in the form of the multiplication table, the
message being constructed by the aid of preconcerted key-words.
Details of the working of these ciphers may be found in the
treatises named in this article. The deciphering of them is one
of the most difficult of tasks. A method of this kind is explained
in the Latin and English lives of Dr John Barwick, whose
correspondence with Hyde, afterwards earl of Clarendon, was
carried on in cryptography. In a letter dated 20th February
1659/60, Hyde, alluding to the skill of his political opponents
in deciphering, says that “nobody needs to fear them, if they
write carefully in good cyphers.” In his next he allays his
correspondent’s apprehensiveness as to the deciphering of their
letters.

“I confess to you, as I am sure no copy could be gotten of any
of my cyphers from hence, so I did not think it probable that they
could be got on your side the water. But I was as confident, till
you tell me you believe it, that the devil himself cannot decypher
a letter that is well written, or find that 100 stands for Sir H. Vane.
I have heard of many of the pretenders to that skill, and have
spoken with some of them, but have found them all to be mountebanks;
nor did I ever hear that more of the King’s letters that
were found at Naseby, than those which they found decyphered,
or found the cyphers in which they were writ, were decyphered.
And I very well remember that in the volume they published there
was much left in cypher which could not be understood, and which
I believe they would have explained if it had been in their power.”

An excellent modification of the key-word principle was constructed
by Admiral Sir Francis Beaufort.

Ciphers have been constructed on the principle of altering
the places of the letters without changing their powers. The
message is first written Chinese-wise, upward and downward, and
the letters are then combined in given rows from left to right.
In the celebrated cipher used by the earl of Argyll when plotting
against James II., he altered the positions of the words.
Sentences of an indifferent nature were constructed, but the
real meaning of the message was to be gathered from words,
placed at certain intervals. This method, which is connected
with the name of Cardan, is sometimes called the trellis or cardboard
cipher.

The wheel-cipher, which is an Italian invention, the string-cipher,
the circle-cipher and many others are fully explained,
with the necessary diagrams, in the authorities named above—more
particularly by Klüber in his Kryptographik.
(J. E. B.)